1. Field of the Invention
The present invention relates to a circuit for protecting a switching power source from an overload condition or a short circuit condition by detecting a load current of the switching power source or a current flowing in the primary winding of a transformer included in the switching power source.
2. Description of the Prior Art
Recently, switching power sources have been commonly brought into use because they are a highly efficient means for providing a stable power supply. There are many types of switching power sources. As described in "Transistor Technology Special No. 28", the switching power sources are classified as single type vs. push-pull types and as self-excited types vs. separate-excited types in terms of power transistor's circuit configuration. To attain stabilized control, PWM types, RCC types or the like are used.
Generally, these circuits are designed to have protective characteristics for protecting the power transistor or the power FET coupled to the transformer from being damaged when a switching power source is overloaded or when the output of the circuit is short circuited.
A protective circuit for protecting from the overload condition or the short circuit condition is described in Japanese Patent Publication Kokoku No. HEI-1-20808, wherein with respect to a switching power source with a PWM feedback control, a current flowing in a power transistor or a power FET is interrupted using a bistable multivibrator when an overcurrent flows in the load for a predetermined period of time.
In the protective circuit as described in the above publication, once an overcurrent is detected, the bistable multivibrator is set so that current does not flow in the power transistor or the power FET. Even after the overcurrent stops flowing, the protected component, that is, the power transistor or power FET will remain OFF until the protective circuit is manually reset. Because there is no automatic reset function, the protective circuit is very troublesome to use. In addition, an overcurrent condition is not detected unless it continues for the predetermined period of time. Therefore, response is slow for detecting continuous short circuits. Also, the circuit has many components and is therefore complicated and expensive to produce.